Methods and circuits for sensing isolated power converter output voltage across the isolation barrier

ABSTRACT

A control circuit for an isolated power converter includes a first sensing circuit that senses a secondary side output voltage and produces a pulse wave modulation (PWM) signal having a duty cycle that is proportional to a value of the secondary side output voltage. The PWM is transferred across the converter isolation barrier to the primary side, and a primary side circuit receives the PWM signal and outputs a control signal. A controller determines the value of the secondary side output voltage from the control signal and uses the value to control primary side power switching devices of the isolated power converter to regulate the secondary side output voltage at a selected value.

RELATED APPLICATION

This application claims the benefit of the filing date of ApplicationNo. 63/188,752, filed on May 14, 2021, the contents of which areincorporated herein by reference in their entirety.

FIELD

This invention relates to isolated power converters. More specifically,the invention relates to methods and circuits for accurate sensing ofthe output voltage of isolated power converters and transferring thesensed output voltage across the converter isolation barrier to achieveaccurate output voltage regulation with fast response time.

BACKGROUND

Isolated power converters are used extensively in applications such asbattery chargers, data center power supplies, chargers for devices suchas cell phones, tablets, and laptop computers, etc. An example of anisolated DC-DC converter according to the prior art is shown in FIG. 1 .Referring to FIG. 1 , in an isolated converter the primary (input) sideis isolated from the secondary (output) side, which may be achieved by atransformer T. Regulation of the output voltage Vo is achieved by adigital controller such as a micro-controller unit MCU2. To achievevoltage regulation, a feedback signal such as an error voltage based onthe output voltage Vo is used as an input to the controller MCU2.Because of the need to maintain electrical isolation between the inputside and the output side, an optocoupler OPT1 is typically used totransfer the error voltage Verror 1 on the secondary side to an errorvoltage Verror2 on the primary side, as input to the controller MCU2.However, a limitation of current approaches is that the error voltageVerror2 is not directly proportional to Vo, and as a result preciseoutput voltage information cannot be obtained at the primary side.

SUMMARY

One aspect of the invention relates to a control circuit for an isolatedpower converter, comprising: a first sensing circuit that senses asecondary side output voltage of the isolated power converter andproduces a pulse wave modulation (PWM) signal having a duty cycle thatis proportional to a value of the secondary side output voltage; a firstisolator that transfers the PWM signal across an isolation barrier to aprimary side of the isolated power converter; a first primary sidecircuit that receives the PWM signal from the first isolator and outputsa control signal; and a first microcontroller that determines the valueof the secondary side output voltage from the control signal andcontrols primary side power switching devices of the isolated powerconverter to regulate the secondary side output voltage at a selectedvalue.

In one embodiment the first primary side circuit comprises a filter thatfilters the PWM signal from the first isolator; wherein the controlsignal comprises the filtered PWM signal having a voltage valueproportional to the value of the secondary side output voltage.

In one embodiment the first sensing circuit comprises a comparator thatproduces the PWM signal.

In one embodiment the first sensing circuit comprises a secondmicrocontroller that produces the PWM signal.

In one embodiment the control circuit includes a feedback circuitcomprising: a second sensing circuit that senses the secondary sideoutput voltage of the isolated power converter and uses the sensedsecondary side output voltage and a reference voltage to produce anerror voltage; a second isolator that transfers the error voltage acrossthe isolation barrier to the primary side of the isolated powerconverter; wherein a steady state output voltage is determined using thereference voltage; wherein the first microcontroller calculates a steadystate gain using the steady state output voltage and a steady statevalue of the control signal, and uses the steady state gain to calibratean actual gain of the first sensing circuit to the determine an actualsecondary side output voltage; wherein the first microcontrollercontrols the primary side power switching devices of the isolated powerconverter to regulate the secondary side output voltage at the selectedvalue.

In one embodiment the second sensing circuit comprises an erroramplifier that produces the error voltage.

In one embodiment the first primary side circuit comprises a samplingcircuit including a capacitor that is charged and discharged accordingto a duty cycle of the PWM signal received from the first isolator;wherein the first microcontroller samples a voltage across the capacitorat a sampling time set by a period of the PWM signal received from thefirst isolator; wherein the control signal comprises the sampled voltageacross the capacitor.

In one embodiment the first isolator outputs the PWM signal to the firstmicrocontroller; wherein the first microcontroller measures a logic hightime interval (T_high) of the PWM signal and uses the T_high interval tocontrol the primary side power switching devices of the isolated powerconverter to regulate the secondary side output voltage at the selectedvalue.

In one embodiment the first primary side circuit comprises a switch thatshapes the PWM signal received from the first isolator by reducing afalling time and rising time of the PWM signal.

In one embodiment the first sensing circuit comprises a secondmicrocontroller that produces the PWM signal and a PWM error signal fromthe output voltage; wherein the first isolator transfers the PWM signalacross the isolation barrier; wherein a second isolator transfers thePWM error signal across the isolation barrier; wherein the first primaryside circuit filters the PWM signal and outputs a first control signal;wherein a second primary side circuit comprising a filter filters thePWM error signal and outputs a second control signal; wherein the firstmicrocontroller uses the first control signal and the second controlsignal to control the primary side power switching devices of theisolated power converter to regulate the secondary side output voltageat a selected value.

Another aspect of the invention relates to a method for controlling anisolated power converter, comprising: sensing a secondary side outputvoltage of the isolated power converter using a first sensing circuitthat produces a pulse wave modulation (PWM) signal having a duty cyclethat is proportional to a value of the secondary side output voltage;using a first isolator to transfer the PWM signal across an isolationbarrier to a primary side of the isolated power converter; using the PWMsignal received from the first isolator at the primary side to produce acontrol signal; and using a first microcontroller to determine the valueof the secondary side output voltage from the control signal and usingthe value of the secondary side output voltage to control primary sidepower switching devices of the isolated power converter to regulate thesecondary side output voltage at a selected value.

In one embodiment the control signal is produced using a first primaryside circuit comprising a filter that filters the PWM signal from thefirst isolator; wherein the control signal comprises the filtered PWMsignal having a voltage value proportional to the value of the secondaryside output voltage.

In one embodiment the secondary side output voltage is sensed using afirst sensing circuit comprising a comparator that produces the PWMsignal.

In one embodiment the secondary side output voltage is sensed using afirst sensing circuit comprising second microcontroller that producesthe PWM signal.

In one embodiment the method comprises using a second sensing circuitthat senses the secondary side output voltage of the isolated powerconverter and uses the sensed secondary side output voltage and areference voltage to produce an error voltage; using a second isolatorto transfer the error voltage across the isolation barrier to theprimary side of the isolated power converter; wherein a steady stateoutput voltage is determined using the reference voltage; wherein thefirst microcontroller calculates a steady state gain using the steadystate output voltage and a steady state value of the control signal, anduses the steady state gain to calibrate an actual gain of the firstsensing circuit to the determine an actual secondary side outputvoltage; wherein the first microcontroller controls the primary sidepower switching devices of the isolated power converter to regulate thesecondary side output voltage at the selected value.

In one embodiment the second sensing circuit comprises an erroramplifier that produces the error voltage.

In one embodiment the first primary side circuit comprises a samplingcircuit including a capacitor that is charged and discharged accordingto a duty cycle of the PWM signal received from the first isolator;wherein the first microcontroller samples a voltage across the capacitorat a sampling time set by a period of the PWM signal received from thefirst isolator; wherein the control signal comprises the sampled voltageacross the capacitor.

In one embodiment the first isolator outputs the PWM signal to the firstmicrocontroller; wherein the first microcontroller measures a logic hightime interval (T_high) of the PWM signal and uses the T_high interval tocontrol the primary side power switching devices of the isolated powerconverter to regulate the secondary side output voltage at the selectedvalue.

In one embodiment the method comprises shaping the PWM signal receivedfrom the first isolator at the primary side by reducing a falling timeand rising time of the PWM signal.

In one embodiment the first sensing circuit comprises a secondmicrocontroller that produces the PWM signal and a PWM error signal fromthe output voltage; wherein the first isolator transfers the PWM signalacross the isolation barrier; wherein a second isolator transfers thePWM error signal across the isolation barrier; wherein the first primaryside circuit filters the PWM signal and outputs a first control signal;wherein a second primary side circuit comprising a filter filters thePWM error signal and outputs a second control signal; wherein the firstmicrocontroller uses the first control signal and the second controlsignal to control the primary side power switching devices of theisolated power converter to regulate the secondary side output voltageat a selected value.

Another aspect of the invention relates to an isolated power convertercomprising a control circuit as described herein.

In one embodiment, the converter output voltage is converted into a PWMsignal with which the duty cycle of the PWM signal is proportional tothe output voltage, the PWM signal is transferred from one side of theisolation barrier to the other side of the isolation barrier and theduty cycle is maintained same, and the DC value of the PWM signal isretrieved by a low pass filter.

In one embodiment, an error amplifier feedback loop is used to regulatethe converter output voltage to its steady state, and the output of thePWM Vo sensing circuit is calibrated by the error amplifier feedbackloop to remove the tolerance in the analog PWM Vo sensing circuit.

In another embodiment, the converter output voltage is converted into adigital signal with which the time interval of logic high isproportional to the output voltage, the digital signal is transferredfrom one side of the isolation barrier to another side of the isolationbarrier and the time interval of the logic high is maintained the same,and the time interval of the logic high signal is calculated (i.e.,retrieved) by a digital controller.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show more clearlyhow it may be carried into effect, embodiments will be described below,by way of example, with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of an isolated inductor-inductor-capacitor(LLC) converter operating as DC to DC converter with a controller at theprimary side, according to the prior art.

FIG. 2 is a schematic diagram of an output voltage Vo sensing circuit,according to one embodiment.

FIG. 3 shows plots of typical waveforms of the output voltage sensingcircuit shown in FIG. 2 .

FIG. 4 is a schematic diagram that may be used to generate a sawtoothwaveform, according to the prior art.

FIG. 5 is a schematic diagram of a PWM Vo sensing circuit having anerror amplifier circuit for calibrating a steady state value, accordingto one embodiment.

FIG. 6 is a schematic diagram of a PWM Vo sensing circuit having anerror amplifier circuit for calibrating a steady state value, accordingto another embodiment.

FIG. 7 is a schematic diagram of an output voltage sensing circuit usinga digital controller at the primary side and another digital controllerat the secondary side, according to one embodiment.

FIG. 8 is a schematic diagram of an output voltage sensing circuit usinga digital controller at the primary side and another digital controllerat the secondary side, according to another embodiment.

FIGS. 9A and 9B are plots showing typical waveforms to detect a T_hightime interval for high and low output voltages, respectively, for theembodiment of FIG. 8 .

FIGS. 9C and 9D are plots showing typical waveforms to detect a T_hightime interval for high and low output voltages, respectively, accordingto another embodiment.

FIG. 10 is a schematic diagram of a Vo sensing circuit, according to oneembodiment.

FIG. 11 is a schematic diagram of a Vo sensing circuit, according to oneembodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows an isolated DC to DC converter with full bridgeinductor-inductor-capacitor (LLC) converter as the power stage,according to the prior art. It is noted that a transformer T is used toachieve isolation between the primary side (shown on the left side ofthe dotted line) and the secondary side (shown on the right side of thedotted line). Np refers to the transformer primary winding and Ns1, Ns2refer to transformer secondary windings.

MCU2 in FIG. 1 refers to a microcontroller unit. It is a digitalcontroller that is used to control the operation of the LLC converter byproviding switching signals (G_(Q1)-G_(Q4)) to the power switches Q₁-Q₄of the power converter primary side. In this circuit, output voltage Voregulation is achieved by transferring the value of error voltageVerror2 from the secondary side across the isolation barrier to primaryside, for use by the digital controller MCU2.

In FIG. 1 MCU2 is placed at the primary side of the power supply. Theoutput voltage Vo at the secondary side passes through a resistordivider circuit (Rb1, Rb2) connected to a voltage regulator TL431, whichsets a reference voltage Vref and includes an OpAmp. The output of theTL431 (Verror1) changes and therefore, the current through the diode ofthe analog optocoupler OP1 changes. The output current of the analogoptocoupler OP1 (i.e., the current through the bipolar junctiontransistor (BJT)) is proportional to its input current (i.e., thecurrent through the diode), so that the error voltage at the primaryside, Verror2, also changes. Verror2 is converted into digital value byan Analog to Digital Converter (ADC) in MCU2 through pin ADC21. The MCU2processes the error information and generates a control signal, such asswitching frequency of the LLC converter, through four gate drivesignals G_(Q1)-G_(Q4). In some cases, the resonant current and inputvoltage are also sensed and used by MCU2.

It is noted that Verror2 contains the error information of the outputvoltage. In the case shown in FIG. 1 , Verror2 depends on thecompensation network (R2, C2), the reference voltage Vref of TL431, theCurrent Transfer Ratio (CTR) of the analog optocoupler OP1, and valuesof resistors R1, R3, Rb1, and Rb2, as shown in equation (1)Verror2=f(Vref,R2,C2,CTR,Rb1,Rb2)  (1)wherein f is typically a complex function.

However, according to the prior approach of FIG. 1 , Verror2 is notdirectly proportional to Vo, as shown below:Verror2≠k Vo  (2)In the above equation, ≠ means “not equal”, and k is a constant.

Described herein are methods and apparatus, i.e., circuits, thatovercome the limitations of prior approaches (such as that shown in FIG.1 ) for transferring sensed analog voltages across the isolation barrierof isolated converters. Embodiments accurately sense and transfer avoltage from one side of the isolation barrier to the other side of theisolation barrier. For example, embodiments may sense the output voltageof an isolated power supply and transfer the sensed voltage from thesecondary side, across the isolation barrier, to the primary side.Although embodiments may be used for any isolated converter application,they are particularly suitable for applications where precise outputvoltage information is required at the other side of the isolationbarrier. According to embodiments, this may be achieved by transferringa signal that is directly proportional to the value of the outputvoltage from one side of the isolation barrier (secondary side) to theother side of isolation barrier (primary side), as shown below:Vop=kVo  (3)

In the above equation, Vop is a DC signal that is the input to a primaryside controller, Vo is the output voltage, and parameter “k” is aconstant. Thus, Vop is directly proportional to the output voltage.

As used herein, the term “isolated converter” refers to an isolated ACto DC, DC to DC, DC to AC, or AC to AC converter. An isolated convertermay be based on a design selected from, but not limited to, full bridge,flyback, inductor-inductor-capacitor (LLC), inductor-capacitor-capacitor(LCC), etc.

FIG. 2 is a circuit diagram of a sensing and control circuit for anisolated converter with a primary side controller, according to oneembodiment. The primary side controller may be, e.g., a digital signalcontroller (DSC), a digital controller such as a microcontroller unit(MCU), a field programmable gate array (FPGA), etc. An example of asuitable controller is DSPIC33CK128MP205, available from MicrochipTechnology Inc., Chandler, Ariz., USA.

For example, the embodiment of FIG. 2 may be used with an isolated DC-DCconverter with full bridge LLC converter as the power stage, as shown inFIG. 5 . The embodiment of FIG. 2 provides Vop as the input to a primaryside controller, e.g., the ADC input to MCU2, as shown in FIG. 2 . TheVop may be used by the primary side controller to regulate the outputvoltage Vo, for example, to achieve output over voltage protection (OVP)and output under voltage protection (UVP), and/or, for example, todetect the change rate of the output voltage, based on which to expeditea dynamic response of the power supply closed loop system. Theembodiment of FIG. 2 is based on a pulse width modulation (PWM)approach.

PWM Vo Sensing

Referring to FIG. 2 , the circuit includes a PWM Vo sensing circuitincluding resistor divider Rb3, Rb4 and a comparator with sawtoothwaveform to convert the output voltage into a PWM signal, an optocoupleror digital isolator OP2 to transfer the PWM signal across the isolationbarrier, and a primary side filter to recover the PWM information. Theoutput voltage Vo is reduced by the resistor divider Rb3, Rb4 to produceVo1. The comparator compares Vo1 with the sawtooth voltage Vsw atfrequency Fsaw and period Tsaw=1/Fsaw. An example of a circuit that maybe used to generate Fsaw is shown in FIG. 4 . The output of thecomparator is a logic signal with a logic high time interval of T_high,as defined by following equation:T_high=Vo1/Vpk*Tsaw  (4)In the above equation, Vpk is the peak voltage of the sawtooth signal.The minimum value of the sawtooth signal is zero.

The duty cycle D of the comparator output signal may be defined asD=T_high/Tsaw, then D=Vo1/Vpk  (5)since Tsaw and Vpk are fixed values. Therefore, the time interval T_highis directly proportional Vo1, which is proportional to Vo. Equivalently,the value D is directly proportional to Vo1. By retrieving the value ofD or T_high, the output voltage value Vo can be measured.

The output voltage of the comparator (V_(PWMS)), which may be referredto as a logic signal, drives the optocoupler or a digital isolator OP2.The optocoupler or digital isolator transfers the logic signal from thesecondary side to primary side of the isolated converter with electricalisolation. V_(PWMP) (the output of the optocoupler or digital isolator,OP2) will have same shape as V_(PWMS), as shown in FIG. 3 , which showskey waveforms of the embodiment of FIG. 2 . It is noted that the peakvalue of V_(PWMP) is Vccp, which is the supply voltage applied to theprimary side of the optocoupler or digital isolator OP2.

In this description, the terms “optocoupler” and “digital isolator” areused interchangeably and have the same meaning (that is, they performeffectively equivalent function), unless otherwise indicated. In thedrawings, the symbol for an optocoupler is used for both an optocouplerimplementation and for a digital isolator implementation. For anoptocoupler the signal is transferred from the diode side (i.e., input)to the BJT side (i.e., output). For example, in the case of FIG. 2 , thesignal is transferred from right side to left side of the optocoupler. Adigital isolator may be provided as an integrated circuit (IC) and doesnot use optical components to transfer the signal across the isolationbarrier, and typically has a faster response time than an optocoupler.

In the embodiment of FIG. 2 , a low pass RC filter implemented with RF1,CF1 and RF2, CF2 is used to filter out the frequency component of Fsawand retrieve the average value of V_(PWMP). The average value is Vop,which may be expressed as:Vop=D*Vccp=Vo1*Vccp/Vpk=Vo*Rb4/(Rb3+Rb4)*Vccp/Vpk  (6)Or Vop=Gain_Vo_PWM*Vo  (6.1)And Gain_Vo_PWM=Rb4/(Rb3+Rb4)*Vccp/Vpk  (6.2)where Gain_Vo_PWM is the equivalent gain of the PWM Vo sensing.Therefore, Vop is directly proportional to the output voltage Vo. Theprimary side digital controller MCU2 senses Vop by an Analog to DigitalConverter (ADC). With this embodiment, the value of the output voltageVo is transferred from the secondary side to the primary side of theisolated converter.

In practical implementations, component tolerances and non-idealoperation of an electronic circuit will introduce errors in the aboveanalysis. For example, due to component tolerances, the peak value ofthe sawtooth signal (Vpk) may vary from design parameters. It can beobserved from equation (6) that if Vpk changes by 5%, Vop will alsochange by about 5%. Therefore, the tolerance of Vop could be high, suchas 5% to 20% in a practical implementation.

Assuming that for a given output voltage value, such as Vo=30V, theaccurately sensed Vop is 3V (Vop=3V) without consideration of componenttolerance. The gain of the PWM Vo sensing circuit is 0.1,Gain_Vo_PWM=Vop/Vo=0.1. If tolerance of 5% is considered, Vop may changefrom 3V to 3V+/−5%=2.85V to 3.15V. If tolerance of 10% is considered,Vop may change from 3V to 3V+/−10%=2.7V to 3.3V. This means that whenthe actual Vo is 30V, the sensed Vop may be between 2.7V to 3.3V.

It is noted that in most applications, Vo sensing tolerance of less than3% is desired and less than 5% is required. Therefore, considering thetolerance of the conversion circuit, such as the Vpk value of thesawtooth signal, the voltage Vop at the primary side obtained by the PWMVo sensing circuit might not have high enough accuracy to tightlyregulate the output voltage Vo.

Therefore, according to another embodiment, the accuracy of the PWM Vosensing circuit may be significantly improved by automaticallycalibrating its steady state operating point with the output voltagefeedback loop using an error amplifier. For example, an error amplifiermay be implemented together with an analog optocoupler (OP1), as shownin FIG. 5 . A suitable device is a three-terminal adjustable shuntregulator with a voltage reference and internal error amplifier, such asTL431 available from Texas Instruments Inc. (Dallas, Tex., USA),although other devices may also be used.

As shown in FIG. 5 , the error amplifier and feedback compensationnetwork (R2 and C2) are used to regulate the output voltage Vo to itssteady state value. The error voltage Verror1 is transferred from thesecondary side to the primary side of the converter as Verror2 using theanalog optocoupler (OP1). Using the error amplifier circuit, thetolerance of the output voltage regulation is determined by thetolerance of the Vref of the error amplifier, and the tolerances of Rb1and Rb2. In the case of TL431, the tolerance of Vref is less than 1%.The tolerances of Rb1 and Rb2 may be between 0.1% to 1%. Therefore, thetolerance of Vo regulation may be limited to less than 3%, such as 1% inmost cases.

Under steady state operation, the output voltage Vo is stable and theerror voltage Verror2 is also stable. Verror2 is sampled by an ADC, inthis case ADC21 input of the MCU2. At the same time, the PWM Vo sensingcircuit also produces Vop that is sampled by another ADC, ADC22 input ofMCU2.

Importantly, since at steady state operation, the output voltage Vo ismaintained at a steady state value (Vo_ss), the actual sensed value ofVop (Vop_act) at steady state operation is directly proportional to thesteady state value of Vo_ss, which is a known value. Therefore, theactual gain of the PWM Vo sensing circuit may be calculated as:Gain_Vo_PWM_act=Vop_act/Vo_ss  (7)where Vop_act is the actual measured voltage at the ADC22 input when thecircuit operates at steady state. The actual gain value of the PWM Vosensing circuit at steady state operation can be calculated using theabove equation. Therefore, by using an error amplifier such as in theembodiment of FIG. 5 , the effect of tolerances of the PWM Vo sensingcircuit can be substantially removed.

This embodiment is further described by way of the followingnon-limiting example, with reference to FIG. 5 .

Assume (A) the ideal gain of the PWM Vo sensing circuit is 0.09; (B) theactual measured Vop_act at steady state is 3V; and (C) the steady stateoutput voltage is Vo_ss=30V.

Then the actual gain of the PWM Vo sensing circuit at steady state isGain_Vo_PWM_act=3V/30=0.1. This steady state gain value is used by MCU2to calibrate the sensed Vop to determine the actual output voltage. Forexample, if the measured VopA=3.5V, the actual output voltage will becalculated using the steady state gain value as VoA=3.5V/0.1=35V. It isnoted that without the calibration, VopA of 3.5V will be “interpreted”by MCU2 using the ideal gain value instead, asVoA_interpreted=3.5/0.09=38.89V, while the actual output voltage is 35V.Similarly, using the calibration method, if the measured Vop isVopB=2.8V, the actual output voltage VoB will be calculated using thesteady state gain value as VoB=2.8V/0.1=28V. It is observed that thecalibration method as described above significantly increases the Vosensing accuracy.

Thus, the output voltage Vo at the secondary side can be measured (orcalculated) accurately from the primary side by:Vo=Vop/Gain_Vo_PWM_act  (8)For example, if at steady state, the output voltage is Vo=30V and thevoltage measured by ADC22 of MCU2 is Vop_act=3V, the actual (steadystate) gain is 3V/30V=0.1. Therefore, if the measured Vop is 2.5V, theactual output voltage, Vo, can be predicted accurately by MCU2 at theprimary side as 25V (=2.5V/0.1). Similarly, if measured Vop is 3.5V, theactual output voltage Vo can be predicted as 35V (=3.5V/0.1). With theaccurately measured output voltage, the primary side MCU2 can achieveaccurate output over voltage protection and output under voltageprotection. MCU2 can also perform additional calculations based on theactual output voltage to achieve better closed loop performance.

In some embodiments the calibration process may be performedcontinuously during operation of the power supply. The actual gain ofthe PWM Vo sensing circuit may be updated continuously. For example,immediately after the power supply turns on, the temperature may be ataround 25° C. (room temperature). The peak value of the sawtooth signalis Vpk_25. If the power supply temperature rises to 100° C., the peakvalue of the sawtooth signal may change to Vpk_100, which is differentfrom Vpk_25. The actual gain of the PWM Vo sensing circuit changes fromGain_25 to Gain_100. With the calibration loop including an erroramplifier and analog optocoupler, the actual gain of the PWM Vo sensingcircuit is updated and the actual output voltage Vo can be measuredaccurately by Vop: Vo=Vop/Gain_Vo_PWM_act.

If the LLC converter used is a single stage AC to DC rectifier withPower Factor Correction (PFC), the output voltage Vo will contain asteady state DC voltage super-imposed by a double line frequency ACripple voltage (100 Hz for Europe and Asia and 120 Hz for NorthAmerica). For example, the DC value of Vo may be Vo_DC=30V. The doubleline frequency ripple may be Vo_rip=5V (peak to peak). Assuming gain of0.1, the measured Vop will also contain a DC value of Vop_DC=3V anddouble line frequency AC ripple of Vop_rip=0.5V (peak to peak). In thiscase, the steady state value, or DC value of Vop, Vop_DC, may be used tocalibrate the actual gain of the PWM Vo sense circuit. Vop_DC may becalculated by MCU2 by taking the average of the sampled Vop voltage overthe period of double line frequency (100 Hz or 120 Hz). The actual gainof the PWM Vo sensing circuit with PFC operation can be calculated as:Gain_Vo_PWM_act_PFC=Vop_DC/Vo_DC  (8.1)With the PWM Vo sensing circuit as shown in FIG. 2 together with thecalibration method as shown in FIG. 5 , the instantaneous output voltagecan be sensed accurately, by the following equation:Vo=Vop/Gain_Vo_PWM_act_PFC  (8.2)With accurate output voltage sensing provided by this embodiment, theripple voltage can be sensed accurately and may be used to improve theperformance of the single stage AC to DC rectifier.

It is also noted that two signals related to the output voltage aresensed at the primary side digital controller, MCU2. One is the outputvoltage error signal that is produced by the error amplifier. This errorsignal is not directly proportional to the output voltage. The other isa voltage signal that is directly proportional to the output voltage.

With the PWM Vo sensing circuit as shown in FIG. 5 , a low pass filterconsisting of RF1, CF1 and RF2, CF2 is used to retrieve the averagevalue of the V_(PWMP). It is noted that this low pass filter willintroduce a small time delay of the Vo sensing. If Vo has a very fastchange, the Vop cannot change instantly. For example, if the sawtoothfrequency is Fsaw=100 KHz, RF1=200KΩ, CF1=150 pF, RF2=150KΩ, CF2=100 pF,the delay time is estimated as around 200 us. This means that if Vochanges quickly from 30V to 33V, it will take Vop about 200 us to changefrom 3V to 3.3V. This delay should be considered in the design. In someapplications, this delay is not desired, or even not allowed.

Such delay may be substantially avoided by the embodiment of FIG. 6 ,which shows a fast PWM Vo sensing circuit with very small delay. At thesecondary side, the output voltage Vo is converted into a PWM signalV_(PWMS) using a comparator and sawtooth wave Vsw. T_high is the logichigh time interval of that PWM signal and is proportional to the outputvoltage Vo. The PWM signal is transferred to the primary side by anoptocoupler or digital isolator OP2 as V_(PWMP). At the primary side,V_(PWMP) is input to a fast T_high value retrieval circuit having aconstant current charging circuit and timed sample circuit, as shown inthe dotted box in FIG. 6 .

V_(PWMP) is used to control the on and off operation of the P-channelMOSFET switch (QP2) of the fast PWM Vo sensing circuit. When V_(PWMP) isat logic high level, QP2 is off and the capacitor C1 is charged byconstant current generated by Qp, Dz (Zener diode), Re, and Rb. C1voltage rises linearly. At the end of T_high, the voltage at C1 is Vpk1,as calculated below by equation (9). The falling edge of the V_(PWMP) issent to MCU2 to instruct MCU2 to sample the C1 voltage at the fallingedge time instant. Therefore, Vpk1 is sensed by MCU2. When V_(PWMP)becomes low, QP2 is turned on and the capacitor C1 is discharged tozero. The peak value of C1 (Vpk1) is calculated from the followingequation:Vop=Vpk1=IC1*T_high=T_high*(Vz−0.7)/Re/C1  (9)where Vz is the Zener diode voltage of Dz. Therefore, the peak value ofC1, Vpk1 (=Vop), is proportional to the logic high time duration(T_high) of the PWM signal. When Vo increases, T_high increases, andtherefore, Vpk1 (=Vop) increases proportionally. The gain of the fastT_high value retrieval circuit as shown in FIG. 6 is calculated as:Gain_Vo_PWM_fast=Vpk1/Vo=Rb4/(Rb3+Rb4)*Tsaw/Vpk*(Vz−0.7)/Re/C1  (10)With the fast T_high value retrieval circuit, no low pass filter isused. The output voltage value is sensed for every Tsaw period, or atthe sawtooth frequency, Fsaw. Since the Fsaw may be around 100 kHz to200 kHz, the time delay for Vo sensing is, therefore, around 5 us or 10us. This is much shorter than the time delay of around 200 us needed bythe PWM Vo retrieval circuit with low pass filter, as shown in FIG. 5 .

As discussed above for the embodiment of FIG. 5 , the accuracy of theembodiment of FIG. 6 may be about 5 to 10% due to component tolerances,which may not be accurate enough to regulate the DC value of the outputvoltage in some applications (e.g., <3% may be required). Therefore, aPWM Vo sensing circuit calibration method similar to that describedabove for the embodiment of FIG. 5 may be implemented in the embodimentof FIG. 6 using an error amplifier and a T_high sensing circuit (asshown in FIG. 6 ) wherein the steady state gain=Vpk1_ss/Vo_ss and theactual gain is Vpk1/Vo_ss. The steady-state value of the output voltageVo is regulated by the error amplifier, and the PWM Vo sensing circuitprovides a voltage signal that is directly proportional to the outputvoltage. This provides additional information for the digital controlloop. The calibration process may be carried out continuously and theactual gain of the fast PWM Vo sensing circuit may be updatedcontinuously.

Accurate PWM Vo Sensing

Another embodiment, shown in FIG. 7 , provides an accurate PWM Vosensing circuit that achieves accuracy of 3% or better. In this circuit,another digital controller MCU1 is used at the secondary side. Theoutput voltage Vo is sensed by the ADC input ADC11 of MCU1 and isconverted into a digital value. MCU1 generates a PWM signal (V_(PWMS))based on the sensed output voltage Vo1. Since the ADC of MCU1 is veryaccurate (i.e., it may have a high resolution such as 12 or 16 bits),the duty cycle of V_(PWMS) is accurately proportional to Vo. V_(PWMS)drives the optocoupler or digital isolator (OP2, as shown in FIG. 7 ),and the PWM signal is transferred to the primary side as V_(PWMP). Sincethe delay time of a digital isolator is very small, V_(PWMP) will havesubstantially the same duty cycle as V_(PWMS). Therefore, Vop may becalculated as:Vop=D*Vccp  (11)If Vccp is well regulated (such as with accuracy of about 1%), the valueat Vop (after the low pass filter) is proportional to the duty cycle ofthe PWM signal V_(PWMS). The accuracy of Vop will be very high, such asbetter than 3% to 5%.

Since the ADC of MCU1 is very accurate, Vop at the primary side will beaccurately proportional to the output voltage Vo at the secondary side.An improvement in this implementation is that the conversion from Vo toa PWM signal is achieved by a digital controller that implements an ADCand PWM module, such as, for example, an MCU.

According to this embodiment, since the output voltage Vo at thesecondary side is sensed and accurately converted to a PWM signal at thesecondary side, and accurately transferred at the primary side as Vop, acalibration circuit such as the embodiment described above having anerror amplifier and related circuit, including a second optocoupler ordigital isolator, is not needed. With this embodiment, only oneoptocoupler or digital isolator (OP2) is needed. Vop is used for bothsteady state output voltage regulation and for dynamic regulation, aswell as output over voltage protection, output under voltage protection,etc.

Because of the presence of the low pass filter after the optocoupler ordigital isolator OP2, the response time of the PWM Vo sensing circuitshown in FIG. 7 may be slow due to a time delay between the change ofoutput voltage Vo at the secondary side and the change of Vop at theprimary side. For example, if the frequency of V_(PWMS) is about 100kHz, the delay time will be around 200 us.

One way to address the time delay of the embodiment of FIG. 7 is shownin the embodiment of FIG. 8 . A speed improvement may be achieved byusing a counter function of the digital controller (e.g., MCU2) tomeasure the logic high time interval (T_high) of the V_high_P signalfrom the optocoupler or digital isolator OP2, as shown in FIG. 8 . Inthis embodiment, the output voltage sensed by MCU1 at the secondary sideis converted into a logic high time interval, T_high, as represented byV_high_S in FIG. 8 . This T_high time interval information istransferred to the primary side by OP2 into V_high_P. The logic hightime interval of V_high_P is the same as the logic high time interval ofV_high_S (T_high). The counter in MCU2 counts the time duration of thelogic high time (the time interval between the rising edge and fallingedge of V_high_P). Therefore, the measured value of T_high by MCU2 isproportional to the output voltage Vo.

FIGS. 9A and 9B show typical waveforms to demonstrate operation of theembodiment of FIG. 8 . Referring to FIG. 9A, is assumed that when Vo isat steady state value, such as 30V, MCU1 will produce a logic high timeof 10 us, T_high1=10 us. This signal (V_high_S) is transferred to theprimary side by the optocoupler or digital isolator as V_high_P. TheT_high value is same for V_high_S and V_high_P. The rising edge ofV_high_P is used to start a counter in MCU2 and the falling edge ofV_high_P is used to stop the counter in MCU2. Therefore, the timeinterval T_high can be accurately measured by MCU2. In a typical MCU,the counter period may be about 20 ns. Therefore, the accuracy of theT_high may be 20 ns/10 us=20 ns/10,000 ns=0.2%. This provides accurateVo sensing for all applications of an isolated converter. MCU1 adds alogic low time interval (T_low) between two logic high signals (T_high).This logic low time interval is needed to separate two samples of outputvoltage. It is noted that this logic low time interval, T_low, is onlyneeded to be long enough to distinguish two logic high intervals. For ageneral purpose MCU, T_low of 1 us will be sufficient to distinguish twohigh signals. For example, if T_high=10 us and T_low=2 us, the updaterate of Vo may be every 12 us. Thus, every 12 us a new Vo value will beobtained at the primary side.

FIG. 9B shows typical waveforms when the output voltage is Vo=15V (abouthalf of the steady state value). With the above example, the T_high is 5us and T_low is still 2 us. Then, every 7 us, the Vo measurement can beupdated at the primary side. This asynchronous information transfer isdesirable to reduce the time delay of the output voltage sensing andtransfer. It is noted that the waveforms of FIGS. 9A and 9B are atdifferent frequencies.

In another embodiment, waveforms for T_high and T_low logic levels areproduced at the same frequency, as shown for two different V_highvoltages in the example of FIGS. 9C and 9D. This may be preferred insome applications as the resulting longer time interval betweeninterrupts (counts), relative to the embodiment of FIGS. 9A and 9B,requires less system resources.

In the above embodiments, it is assumed that a high speed optocoupler ora digital isolator is used to transfer signals from the secondary sideto the primary side. However, due to costs, in actual implementations aregular optocoupler or a slow optocoupler may be used as they are lessexpensive than high speed optocouplers. With a slow optocoupler, therising and falling edges of the PWM signal are long, which reduces theaccuracy of Vop. To address this issue, as described herein, Vo sensingcircuits using a regular or slow optocoupler may be implemented with asmall-signal MOSFET after the optocoupler. For example, as shown in FIG.10 , the gate of MOSFET S₁ may be connected to the optocoupler OP2output V_(PWMP) and the drain connected to the supply voltage Vccp viaR12. The drain voltage of MOSFET S₁ (Vds1) will have a sharp rise andfalling edge. Therefore, the average voltage of Vop will follow the dutycycle of the VPWMS very closely.

The embodiment of FIG. 11 is a digital implementation of the circuitshown in FIG. 5 . Two MCUs are used, MCU1 at the secondary side and MCU2at the primary side. The secondary side MCU generates two PWM signals,Error PWM and Vo PWM. The duty cycle of the first PWM signal (Error PWM)is proportional to the output of the error amplifier Verror1 as shown inFIG. 5 , which is an analog signal. In FIG. 11 , the error voltageVerror1 is calculated by the MCU1 as an internal value and is output asa digital signal Error PWM. In the embodiment of FIG. 11 , bothisolators (OP2) may be implemented with optocouplers or digitalisolators, since both signals are PWM signals.

The duty cycle of the second PWM signal (Vo PWM) is proportional to theactual value of Vo. Using the calibration method described above, theError PWM signal may be used to calibrate the Vo PWM signal to removethe possible error introduced because of the inherent delay time of thedigital isolator or optocoupler and to improve the accuracy of the PWMVo sensing.

Similarly, the T_high time detection method as described with respect tothe embodiment of FIG. 8 may also be used to detect the pulse width ofthe Vo PWM signal and therefore to detect the actual output voltage Vo.The details would be apparent to one skilled in the art based on theembodiments described above.

The contents of all cited documents are incorporated herein by referencein their entirety.

EQUIVALENTS

While the invention has been described with respect to illustrativeembodiments thereof, it will be understood that various changes may bemade to the embodiments without departing from the scope of theinvention. Accordingly, the described embodiments are to be consideredexemplary and the invention is not to be limited thereby.

The invention claimed is:
 1. A control circuit for an isolated powerconverter, comprising: a first circuit that senses a secondary sideoutput voltage of the isolated power converter and produces a pulse wavemodulation (PWM) signal having a duty cycle that is proportional to avalue of the secondary side output voltage; a first isolator thattransfers the PWM signal across an isolation barrier to a primary sideof the isolated power converter; a first primary side circuit thatreceives the PWM signal from the first isolator and outputs a controlsignal; a first microcontroller; and a feedback circuit comprising: asecond circuit that senses the secondary side output voltage of theisolated power converter and uses the sensed secondary side outputvoltage and a reference voltage to produce an error voltage; and asecond isolator that transfers the error voltage across the isolationbarrier to the primary side of the isolated power converter; wherein asteady state output voltage is determined using the error voltage;wherein the first microcontroller calculates a steady state gain usingthe steady state output voltage and a steady state value of the controlsignal, and uses the steady state gain to calibrate an actual gain ofthe first circuit that senses the secondary side output voltage todetermine an actual secondary side output voltage; and wherein the firstmicrocontroller controls primary side power switching devices of theisolated power converter to regulate the secondary side output voltageat a selected value.
 2. The control circuit of claim 1, wherein thefirst primary side circuit comprises a filter that filters the PWMsignal from the first isolator; wherein the control signal comprises afiltered PWM signal having a voltage value proportional to the value ofthe secondary side output voltage.
 3. The control circuit of claim 2,wherein the first primary side circuit comprises a switch that shapesthe PWM signal received from the first isolator by reducing a fallingtime and rising time of the PWM signal.
 4. The control circuit of claim1, wherein the first circuit that senses the secondary side outputvoltage comprises a comparator that produces the PWM signal.
 5. Thecontrol circuit of claim 1, wherein the first circuit that senses thesecondary side output voltage comprises a second microcontroller thatproduces the PWM signal.
 6. The control circuit of claim 5, wherein thefirst isolator outputs the PWM signal to the first microcontroller;wherein the first microcontroller measures a logic high time interval(T_high) of the PWM signal and uses the T_high interval to control theprimary side power switching devices of the isolated power converter toregulate the secondary side output voltage at the selected value.
 7. Thecontrol circuit of claim 1, wherein the second circuit that senses thesecondary side output voltage comprises an error amplifier that producesthe error voltage.
 8. The control circuit of claim 1, wherein the firstprimary side circuit comprises a sampling circuit including a capacitorthat is charged and discharged according to a duty cycle of the PWMsignal received from the first isolator; wherein the firstmicrocontroller samples a voltage across the capacitor at a samplingtime set by a period of the PWM signal received from the first isolator;wherein the control signal comprises the sampled voltage across thecapacitor.
 9. The control circuit of claim 1, wherein the first andsecond circuits that sense the secondary side output voltage comprise asecond microcontroller that produces the PWM signal and a PWM errorsignal from the secondary side output voltage; wherein the firstisolator transfers the PWM signal across the isolation barrier; whereinthe second isolator transfers the PWM error signal across the isolationbarrier; wherein the first primary side circuit filters the PWM signaland outputs a first control signal; wherein a second primary sidecircuit comprises a filter that filters the PWM error signal and outputsa second control signal; wherein the first microcontroller uses thefirst control signal and the second control signal to control theprimary side power switching devices of the isolated power converter toregulate the secondary side output voltage at the selected value.
 10. Amethod for controlling an isolated power converter, comprising: using afirst circuit that senses a secondary side output voltage of theisolated power converter and produces a pulse wave modulation (PWM)signal having a duty cycle that is proportional to a value of thesecondary side output voltage; using a first isolator to transfer thePWM signal across an isolation barrier to a primary side of the isolatedpower converter; using the PWM signal received from the first isolatorat the primary side to produce a control signal; using a firstmicrocontroller to determine the value of the secondary side outputvoltage from the control signal; using a second circuit that senses thesecondary side output voltage of the isolated power converter and usesthe sensed secondary side output voltage and a reference voltage toproduce an error voltage; and using a second isolator to transfer theerror voltage across the isolation barrier to the primary side of theisolated power converter; wherein a steady state output voltage isdetermined using the error voltage; wherein the first microcontrollercalculates a steady state gain using the steady state output voltage anda steady state value of the control signal, and uses the steady stategain to calibrate an actual gain of the first circuit that senses thesecondary side output voltage to determine an actual secondary sideoutput voltage; and wherein the first microcontroller controls primaryside power switching devices of the isolated power converter to regulatethe secondary side output voltage at a selected value.
 11. The method ofclaim 10, wherein the control signal is produced using a first primaryside circuit comprising a filter that filters the PWM signal from thefirst isolator; wherein the control signal comprises a filtered PWMsignal having a voltage value proportional to the value of the secondaryside output voltage.
 12. The method of claim 11, comprising shaping thePWM signal received from the first isolator at the primary side byreducing a falling time and rising time of the PWM signal.
 13. Themethod of claim 10, wherein the first circuit that senses the secondaryside output voltage comprises a comparator that produces the PWM signal.14. The method of claim 10, wherein the first circuit that senses thesecondary side output voltage comprises a second microcontroller thatproduces the PWM signal.
 15. The method of claim 14, wherein the firstisolator outputs the PWM signal to the first microcontroller; whereinthe first microcontroller measures a logic high time interval (T_high)of the PWM signal and uses the T_high interval to control the primaryside power switching devices of the isolated power converter to regulatethe secondary side output voltage at the selected value.
 16. The methodof claim 10, wherein the second circuit that senses the secondary sideoutput voltage comprises an error amplifier that produces the errorvoltage.
 17. The method of claim 10, wherein the first primary sidecircuit comprises a sampling circuit including a capacitor that ischarged and discharged according to a duty cycle of the PWM signalreceived from the first isolator; wherein the first microcontrollersamples a voltage across the capacitor at a sampling time set by aperiod of the PWM signal received from the first isolator; wherein thecontrol signal comprises the sampled voltage across the capacitor. 18.The method of claim 10, wherein the first and second circuits that sensethe secondary side output voltage comprise a second microcontroller thatproduces the PWM signal and a PWM error signal from the secondary sideoutput voltage; wherein the first isolator transfers the PWM signalacross the isolation barrier; wherein the second isolator transfers thePWM error signal across the isolation barrier; wherein the first primaryside circuit filters the PWM signal and outputs a first control signal;wherein a second primary side circuit comprises a filter that filtersthe PWM error signal and outputs a second control signal; wherein thefirst microcontroller uses the first control signal and the secondcontrol signal to control the primary side power switching devices ofthe isolated power converter to regulate the secondary side outputvoltage at the selected value.
 19. An isolated power convertercomprising a control circuit, the control circuit comprising: a firstcircuit that senses a secondary side output voltage of the isolatedpower converter and produces a pulse wave modulation (PWM) signal havinga duty cycle that is proportional to a value of the secondary sideoutput voltage; a first isolator that transfers the PWM signal across anisolation barrier to a primary side of the isolated power converter; afirst primary side circuit that receives the PWM signal from the firstisolator and outputs a control signal; a first microcontroller; and afeedback circuit comprising: a second circuit that senses the secondaryside output voltage of the isolated power converter and uses the sensedsecondary side output voltage and a reference voltage to produce anerror voltage; and a second isolator that transfers the error voltageacross the isolation barrier to the primary side of the isolated powerconverter; wherein a steady state output voltage is determined using theerror voltage; wherein the first microcontroller calculates a steadystate gain using the steady state output voltage and a steady statevalue of the control signal, and uses the steady state gain to calibratean actual gain of the first circuit that senses the secondary sideoutput voltage to determine an actual secondary side output voltage; andwherein the first microcontroller controls primary side power switchingdevices of the isolated power converter to regulate the secondary sideoutput voltage at a selected value.